the 780ti wont have any problems with 2560x1080. 2560x1440 is 33% higher pixel count than 2560x1080; 2560x1080 is 33% higher pixel count than 1920x1080. you can roughly expect your fps performance drop to be the same 33% as the pixel count. for the most part my 780 classified handled 2560x1440 without issue and only a few games like crysis 3, gta5, witcher 3 did i need to turn down AA levels along with a couple key settings. so in my opinion your 780ti will do excellent at 2560 ultra wide.
if you are going to go with 3440x1440 then we are talking a whole different story, a whopping 79% more pixels than 2560 ultra wide. you would almost need a second 780ti in sli to give you similar performance at 3440 ultra wide than you would...